Capacitive type proximity sensors are used intensively for distance detection purposes, featuring a flat, mostly round or square, sensing electrode. Guarding electrodes are used to obtain a linear measuring range, which is usually in the order of one-third of the sensor size. The same sensing technology is used for collision prevention purposes on Philips Medical equipment, featuring a measuring range which is larger than the sensor size.
The technique uses capacitive type proximity sensor electrodes along the outer boundaries of the casing of moving (Medical) equipment in order to detect the approaching of an object. In the first place it is meant as a preventive measure against colliding with the patient on the examination table and in the second place with other equipment in the direct environment.
The size and shape of the electrodes of the capacitive type proximity sensor are driven by the size and shape of the casing of the equipment. The sensing range of the detectors extends to distances which are even larger than the capacitive type proximity sensor structure itself. In such cases, the sensor characteristic will certainly not be linear with distance anymore.
A non linear characteristic is basically not a real problem as long as it reproduces nicely. Reproducing non-linear behaviour can only be expected when the sensor geometry remains fixed. The sensor geometry is basically configured by the shape of the two capacitive electrodes and their mutual distance and relative orientation. One electrode of the sensor is the surface of the object/target to be detected, and is thus not defined at all. The shape and size of the sensor electrode is driven by the shape and size of the equipment in question. In that way, there is a limited freedom in selection of sensor electrode shape and size. It is without question that the sensor characteristic shows significant variation, depending on object size and the relative position of the object to the shape of the sensor electrode. The result is that the collision prevention system comes up with stopping distances which vary significantly as well, depending on object size/shape and relative approaching position over the sensor electrode area.
In the present systems, the effective range of stopping distances is acceptable, when calibration steps are done. Sometimes small (mechanical) modifications or an ad-on is made in the sensitive area of the sensors, introducing extra variation. This step brings extra stopping distance change, needing an extra calibration step.